1. Field of the Invention
The present invention relates to a disk drive, and more particularly, to a method for driving a spindle motor using adaptive feedforward control.
2. Description of Related Art
In general, digital data storage devices use disk drives having rotary rigid disks. The technologies related to a disk drive has been developed to increase storage capacity and accuracy of a disk drive while reducing the weight and power consumption of the disk drive. According to this development of technologies, precision control of a rotary disk has become more important.
According to a conventional technology, the speed of a spindle motor of a disk drive is maintained constant through standard control type feedback correction, that is, by supplying steady-state current to the spindle motor using a servo loop. The conventional spindle motor driving method using the feedback control is described below in detail.
A proportional integral digital controller that is typically of a firmware type transmits a control signal to a digital/analog converter. The digital/analog converter converts the control signal to an analog signal and transmits the converted analog signal to a motor pre-driver. The motor pre-driver controls the rotation speed of the spindle motor using the received control signal. Then, the actual speed of the spindle motor is measured by a digital counter and reflected in a speed signal. Next, the speed signal is deducted from a reference signal to generate a speed error signal. The speed error signal is fed back to the proportional integral digital controller so that the rotation speed of the spindle motor can be maintained at a target speed.
FIG. 1 is a graph showing the trace of the speed of a spindle motor at room temperature and extremely low temperature according to a conventional spindle motor driving method. In the graph of FIG. 1, the horizontal axis and the vertical axis denote the lapse of time and the rotation speed of the spindle motor, respectively.
As the temperature of the disk drive goes down below zero, the frictional force of a fluid bearing is gradually increased. In FIG. 1, it can be seen that, when the spindle motor is driven at a temperature lower than the room temperature, the rise of the rotation speed is delayed due to the increase of the frictional force of the spindle motor at around the target rotation speed. Also, when the temperature is extremely lower than the room temperature, a spin error SUM is sharply increased and saturated. A spin DAC indicated in the lower portion of the graph denotes the magnitude of a control signal that is proportional to the size of current applied to the spindle motor. It is noted that the spin DAC value decreases at the time point when the rotation speed of the spindle motor reaches a target RPM. This is because, when the rotation speed of the spindle motor reaches the target RPM, since the spindle motor is no longer accelerated and maintains a constant speed, large current is not needed.
FIG. 2 is a graph showing the case of the extremely low temperature of FIG. 1. In FIG. 2, it can be seen that the time for the spindle motor to reach the target rotation speed is remarkably increased in the low temperature environment than in the room temperature environment. This is because more current is required since the frictional force of the fluid bearing increases in the low temperature environment, and the time to reach the target rotation speed increases. Also, the spin speed error SUM of the controller is radically increased so that a phenomenon that the spin speed error SUM of the motor is saturated. Accordingly, the efficiency in driving of the motor is lowered.
Therefore, the conventional spindle motor driving method cannot prevent the increase of the time for the spindle motor to reach the target rotation speed when the frictional force of the fluid bearing changes according to the change of temperature in the driving environment when the spindle motor is driven. Accordingly, initial disk access time increases.